The present invention pertains generally to antennas and specifically to a commutated antenna array which has particular application for use in automatic direction finding systems.
Numerous automatic direction finding systems exist for providing navigational bearing information with respect to the direction of a radio (RF) signal as received from a transmitter having a known location. Some of these systems use a rotating antenna to physically rotate a cardioid pattern which is used to sinusoidally modulate the amplitude of the RF signal. By comparing the phase of the sinusoidal modulation imparted to the RF signal with that of a locally generated sinusoidal signal of the same frequency, and whose phase remains constant vis-a-vis the antenna angular deviation, the direction of the RF signal can be determined and bearing information ascertained.
Although it is known that a rotating pattern such as a cardioid can be electronically simulated by a commutated antenna array which obviates the need for a rotating antenna, this approach has not been widely adopted. The only such system of which the applicant is aware is one provided by Collins Radio Company of Canada, Ltd. for airborne navigation designated Direction Finder DF-301E (described in Instruction Manual No. 523-1000-429-1011-42) that uses a single antenna consisting of a cavity-backed disk separated from a ground plane by an annular slot having taps around its perimeter which are sequentially activated to generate the rotating cardioid pattern. Although effective for its intended purpose, this slotted antenna lacks the versatility necessary for extended use such as for surface navigation onboard ships. For example, its flat profile (3.6 inches in height) materially reduces its visibility to RF signals which may not significantly impair its operation for airborne navigation where one has the benefit of height but certainly would for surface navigation. Furthermore, the antenna gain which is affected by the size of the ground plane may be unduly limited by dimensional constraints on the antenna so that desired signals having reduced intensities may not be capable of being received and detected. Despite these shortcomings, the good directivity and high integrity for the cardioid pattern generated which are so important to achieve good direction finding performance are compensating factors exhibited in the DF-301E slotted antenna. The latter characteristic is attributable to the intrinsic operation of the slotted antenna, which unlike individual antenna arrays, substantially eliminates unwanted interaction effects created by signal reradiations that would otherwise distort the desired antenna cardioid pattern used in direction finding. Although these effects could be combatted by separating adjacent antennas by greater distances the resultant size of an individual antenna array efficacious for automatic direction finding could be too large to be useful.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved commutated antenna array for amplitude modulating an RF signal such as used for automatic direction finding.
It is a further object of the present invention to provide such a new and improved array which is highly versatile, yet displays all of the characteristics essential for direction finding.
It is still a further object of the present invention to provide such a new, improved and versatile array which may be realized in a small package without detrimental interaction effects.
These objects as well as others, and the means by which they are achieved through the present invention, may best be appreciated by referring to the Detailed Description of the Invention which follows together with the appended drawings.